Hydrocarbon production strata are often composed of limestone, dolomite and other calcerous materials. The production of hydrocarbons from oil and gas wells drilled into these strata is often less than desired. This is the caused by low permeability of the strata which results in poor conductivity to the wellbore. These formations are often fracture-acidized in an attempt to improve the conductivity. This is accomplished by injecting an aqueous acid solution into the wellbore, causing a fracture in the production strata which provides a permeable, conductive channel from the strata to the wellbore.
The aqueous acid normally used in this operation is hydrochloric acid, in concentrations ranging from 3% to 28%, for stimulation of carbonate formations. In using hydrochloric acid, the reaction of acid with carbonate rock occurs at such a rate that the acid is usually depleted rapidly. This rapid depletion of acids results in a limited depth of penetration of acid into into the formation. To increase the depth of penetration of acid into the formation, acids have been retarded by adding a suitable viscosifying agent, chemical retarders, foaming the acid, emulsifying the acid, or by cross-linking the acid viscosifying agent. Each method of retardation has shown limited use in field applications. These methods of retardation generate fluids of low viscosities and have a low degree of viscous stability (viscosities changing with time) especially with increasing reservoir temperature.
It is advantageous to initially viscosify the fracture-acidizing fluid. The fluid's viscosity is proportionally related to the created fracture geometry and fracture width so that more viscous fluids will produce longer and wider fractures. In addition, the viscosity of the acid will decrease the reaction rate or retard the acid etching, allowing high strength acid to penetrate deep into the fracture during injection. After the viscous acid is injected into the fracture, the viscosity should diminish at a desirable rate to allow the acid to etch the fracture faces. Stable viscosity and control thereof are a problem.
Fracture-acidizing fluids have been previously thickened with synthetic polymers, derivatives of natural materials such as carboxymethyl cellulose, guar, hydroxypropyl guar, biopolymers such as xanthan gum, and the like. The apparent viscosity of such thickened acid fluids is often low and decreases rapidly in strong acids due to the hydrolysis of the polymer especially at higher temperatures i.e., 130.degree. F. Synthetic polymers also often precipitate from solution in the presence of high concentrations of calcium and magnesium ions. Such precipitants tend to plug the formation which can be detrimental to the recovery of the oil and natural gas from the reservoir. Improved thickened fracture-acidizing compositions are an objective of this invention that are effective at the higher operating temperatures found in deep wells.